Gas monitors are known that include one or more replaceable amperometric gas sensors providing an electrical current the magnitude of which provides a measure of the amount of gas detected in an atmosphere. This signal is analysed by circuitry within the monitor to give a monitor output which may be in the form of a display (in an analogue or digital form) of the amount of a specific gas detected and/or the output may be a printer or plotter and/or an alarm to give an audible and/or visual warning if the concentration of a gas falls to an undesirable level or if the concentration of a gas arises above a certain threshold level; the output signal could be recorded for subsequent analysis. However, the gas monitor need not necessarily give a direct indication of the amount of gas detected but may use the signal from one or more sensors to compute another parameter which may be displayed or printed; thus, the signal from a gas sensor may be used, together with other measurements, to compute the efficiency of a boiler (see British Patent No. 2,064,780).
The sensors used in the monitors of the present invention are amperometric electrochemical sensors of the type having a sensing (or working) electrode which is in communication with the atmosphere being sensed, a counter electrode and a reference electrode and all three electrodes are in contact with electrolyte within the sensor and are connected via respective terminals to the circuitry within the monitor; sensors of this type will be eferred to herein as "three electrode sensors". The potential difference between the sensing electrode and the reference electrode may be controlled and in some sensors this is done by connecting these two electrodes to the inputs of an operational amplifier either directly or through a resistor, e.g. see British Patent Specification Nos. 1,101,101 and 1,385,201 and U.S. Pat. No. 3,776,832 and European Patent Application No. 0,220,896A.
A circuit generally in accordance with the above patents is set out in FIG. 1 of the accompanying drawings. In FIG. 1, the sensor is indicated by the general reference number 10 and includes an electrolyte (sulphuric acid), a sensing electrode 12, a reference electrode 14 and a counter electrode 16 all of which are in contact with the electrolyte. The sensing electrode and the reference electrode are joined via terminals 12a and 14a to respective inputs of an operational amplifier 18 whose output is connected to the counter electrode 16 via terminal 16a. A resistor 20 is present between the sensing electrode 12 and its input to the operational amplifier 18. The sensing electrode 12 is in contact with an atmosphere that is being monitored and when the atmosphere contains a gas of the type being detected, this gas undergoes an electrochemical reaction which depolarizes the sensing electrode 12 causing the potential of that electrode to alter and so cause an imbalance between the potential of the sensing electrode 12 and the reference electrode 14 and hence between the inputs of the operational amplifier 18. The potential difference between the operational amplifier inputs causes the operational amplifier to supply current through its output to counter electrode 16 and hence causes a current to flow in the sensor cell 10 between the counter electrode 16 and the sensing electrode 12 (however substantially no current flows between the reference electrode and the sensing electrode). The current flowing through the sensor cell, which is directly related to the amount of gas in the atmosphere, can be measured, for example, by including a resistor between the amplifier output and the counter electrode 16 and measuring the voltage drop across the resistor (see U.S. Pat. No. 3,776,832); alternatively, the current flowing through the cell may be measured by a current follower connected to line 22 or by measuring the potential difference across a resistor between line 22 and a ground or other fixed potential (see European Patent Application No. 0,220,896); alternatively, the voltage drop across the resistor 20 may be measured (see British Patent No. 1,101,101). Resistor 20 is included between the sensing electrode and the operational amplifier in order to slow the response time of the sensor and thus provide immunity from electronic noise and fluctuations in the potential of the sensing electrode; the value of resistor 20 is generally chosen between 0 and 500 ohms.
The amplifier 18 has an offset null potentiometer 19 which is usually set such that current is supplied by the amplifier to its output unless there is no potential difference between the two inputs of the amplifier; however, the offset null can be set to provide an offset voltage between the amplifier inputs, in which case current is supplied by the amplifier to its output unless the potential between the amplifier inputs is a certain, non-zero value (the offset voltage). When an offset voltage is set, the amplifier will supply current to its output and to the sensor until a potential difference is created between its inputs that equals the offset voltage and when this occurs, there is a potential difference between the sensing electrode and the electrolyte immediately surrounding it; the presence of a layer of electrolyte around an electrode that is at a different potential to the electrode is termed a `double layer` and acts like a capacitor.
It is usual to adjust the potentiometer 19 to trim the offset null of the amplifier 18 to substantially zero so that, when the monitor is switched on, there is no voltage difference between the reference electrode and the sensing electrode and so there is no charging of the double layer on the sensing electrode which would give a spurious reading when the monitor is first switched on. In some monitors a substantial voltage offset is maintained between the sensing and reference electrodes in order to minimise cross-sensitivity with gases other than the gas that it is desired to detect which may be present in the atmosphere being monitored and such an arrangement is used in particular to minimise cross-sensitivity to hydrogen gas; however when a large offset is established between the sensing and the reference electrode, it can take 2 days for a sensor to settle to a steady state when the monitor is first switched on (and no accurate readings can be taken during this period) and such monitors are normally kept permanently switched on to avoid this problem.
If the sensor, which is a replaceable item, is not correctly installed in the monitor, e.g. if the electrode terminals of the sensor do not properly contact the corresponding terminals of the monitor, the monitor will show a zero reading, even if there is gas in the atmosphere being detected and the user will assume that the atmosphere contains no gas of the type being monitored. Since monitors can detect poisonous gases, e.g. carbon monoxide, this can be dangerous. Thus, the circuit of the above type is not `fail-safe`. A further problem can arise if the electrolyte of the sensor cell 10 evaporates so that the cell dries out and no longer conducts electric current; in this case, a zero reading is again obtained even if there is gas in the atmosphere being monitored.
It has been proposed in CH-636 447 to check that a pH reference (or counter) electrode is operating properly in a potentiometric titration apparatus by measuring the resistance between the counter and the sensing electrodes of the titration apparatus by passing an alternating current between these electrodes, rectifying the resulting current, passing an identical alternating current through a fixed resistor, rectifying the resulting current and subtracting the two rectified currents from each other to form a signal and stopping the titration if the said signal indicates that the resistance between the sensing electrode and the counter electrode is either much higher or much lower than the resistance of the fixed resistor. Apart from being applied in a different technical field to the present invention (CH-636 447 uses potential as a sensing criterion rather current as is the case with the present invention), the proposal in CH-636 447 is complex and hence expensive to implement.
EP 0 039 549 describes a system for testing the operation of electrochemical gas sensors having a salt solution electrolyte by applying a pulse of potential between the electrodes of the sensor that decomposes the electrolyte; if there is a resulting change in current flowing through the sensor, then the sensor is operating properly. The electrolysis of the electrolyte causes gas to be generated, which is disadvantageous in a sealed gas sensor since it can affect the sensor operation and pressure can build up within the sensor causing it to leak; the present provides a system that avoids these problems.